CODEWITHSUNDEEP
pythonpython-3.xmatplotlibscipyfft
Ingeneravit
Ingeneravit

Reputation: 23

Amplitude units in FFT

I'm completely new to python, scipy, matplotlib and programming in general. I'm using the following code, which I came across online, to apply FFT to .wav files:

import scipy.io.wavfile as wavfile
import scipy
import scipy.fftpack as fftpk
import numpy as np
from matplotlib import pyplot as plt

s_rate, signal = wavfile.read("file.wav")

FFT = abs(scipy.fft.fft(signal))
freqs = fftpk.fftfreq(len(FFT), (1.0/s_rate))

plt.plot(freqs[range(len(FFT)//2)], FFT[range(len(FFT)//2)])
plt.xlabel('Frequency (Hz)')
plt.ylabel('Amplitude')
plt.show()

The resulting graphs give amplitude values that range from 0 to a few thousands, depending on the files, and I have no idea what unit these are in. I'm guessing they might be relative amplitudes, and I was wondering if there is a way to turn that into decibels, as I need specific values.

Thank you

Tanguy

Upvotes: 0

Views: 1943

Answers (1)

hotpaw2
hotpaw2

Reputation: 70703

They are amplitudes relative to the quantization units used for the samples in your input signal. So, without calibrating your input signal against a known level of source input (to get Volts per 1 bit change, etc.), the actual units are unknown. If calibrated, you may still need to divide the magnitudes of the FFT output by N (the FFT length), depending on your particular FFT implementation.

To get Decibels, convert by taking 20*log10(abs(...)) of the FFT results, and offset by your 0 dB calibration level.

Upvotes: 2

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